PROJECT SUMMARY Current evidence suggests that there are unique benefits of breastfeeding for mothers receiving medication- assisted treatment (MAT) for opioid use disorder (OUD) and their infants. Breastfeeding rates, however, remain low among mothers in treatment for OUD and detailed information on determinants of infant feeding decisions and behaviors in this population is lacking. This knowledge gap is significant and precludes explanations for suboptimal breastfeeding rates and targets for interventions designed to increase breastfeeding intention and subsequent lactation to ultimately improve health outcomes. Our objective here is to fill the gap in the existing knowledge and literature regarding influences of breastfeeding decisions and behaviors in women in treatment for OUD. We plan to attain our objective by pursuing the following aims: 1) to assess determinants of breastfeeding intention in the prenatal period; 2) to assess determinants of breastfeeding initiation in the postpartum period; and 3) to assess determinants of breastfeeding continuation at three months postpartum. To address our aims, we plan to conduct a two year research project that will enroll pregnant women receiving MAT for OUD and follow them longitudinally until three months after giving birth. Survey and medical chart data will be collected at three time points using standardized survey procedures and data abstraction techniques. Data will be compared between women who report intention to breastfeed and women who do not report intention to breastfeed, between women who report breastfeeding initiation and women who do not report breastfeeding initiation, and between women who continue to breastfeed for at least 3 months and women who do not. Study findings will have implications for innovations in breastfeeding promotion and will be immediately relevant to pregnant women with OUD and the practitioners, policymakers, and healthcare systems that serve such women. This is notable given that the rapid growth in maternal opioid use and misuse and associated neonatal consequences present a significant and ongoing public health burden in the United States. Through the proposed research, we expect to contribute novel data that will inform our long-term plans to design future breastfeeding promotion interventions that are uniquely tailored toward women undergoing treatment for OUD, and to guide new approaches for clinical and treatment professionals to discuss breastfeeding with their these women, thereby setting the stage for improved maternal and infant health.